1. Field of the Invention
This invention relates to microwave/millimeter wave circuits, and more particularly to circuit structures in this wavelength range that employ discrete circuit elements.
2. Description of the Related Art
The microwave/millimeter wave spectrum is generally defined as extending from about 1 m at 300 MHz to about 1 mm at 300 GHz, with the millimeter wave portion of the spectrum covering about 30-300 GHz.
Microwave/millimeter wave circuits have been developed for numerous applications, such as telephone transmission, microwave ovens, radar, and automotive uses that include collision warning radar, millimeter wave imaging systems, blind-spot radar, toll exchanges and radar for monitoring road surfaces. They have generally been implemented as either hybrid circuits, with discrete elements wire bonded to a circuit board substrate, or as monolithic microwave integrated circuits (MMICs) in which active circuit components are integrated on the same wafer as passive components. While the development of microwave devices such as collision warning radars has been based upon the use of MMICs, several factors make the fabrication of MMICs considerably more expensive than fabricating discrete devices.
MMICs are much larger than discrete devices and accordingly take up a larger area of the semiconductor wafer; the cost of a chip depends heavily upon its size. Also, due to the complexity of fabricating MMICs, their typical yield is on the order of 15%-30%, as opposed to yields of more than 80% that can easily be achieved with discrete devices. For example, from a 7.6 cm (3 inch) wafer it is possible to obtain approximately 14,000 working discrete devices, as compared to approximately 400 MMICs.
The use of flip-chip bonding techniques, in which conductive contact "bumps" are provided on the circuit face of a chip which is "flipped" and both electrically and mechanically affixed to a circuit board via the bumps, is disclosed in H. Sakai et al., "A Novel Millimeter-Wave IC on Si Substrate Using Flip-Chip Bonding Technology", IEEE MTT-S Digest, 1994, pages 1763-1766. In this publication, millimeter wave heterojunction transistors are flip-chip bonded to microstrip lines formed on a silicon substrate. However, silicon has very high losses at millimeter wave frequencies. To overcome these losses, Sakai et al. deposited 9 micrometer thick SiO.sub.2 on the silicon substrate to use in the fabrication of microstrip transmission lines. However, the loss for 50 ohm transmission lines at 60 GHz was still 0.55 dB/mm, which is too high for low-loss matching elements, power combiners and couplers. In fact, the circuit that was reported to have been constructed for testing operated at 20 GHz rather than 60 GHz, presumably because of excessive transmission line losses at the higher frequency. Furthermore, silicon as well as the ceramic materials that are conventionally used for circuit boards at lower frequencies are relatively expensive.
Sakai et al. also proposed a bump technology that is based upon the use of an insulate resin with no heating. This results in a relatively low accuracy of device placement, which is a critical factor at the high frequencies involved in microwave/millimeter wave circuits. In fact, a primary reason for the movement towards MMICs as opposed to hybrid circuits is the high cost of hybrid circuits that results from the need to hand tune each circuit; this process is both time consuming and expensive. The higher the frequency, the smaller is the length of matching elements (transmission lines), and the more sensitive is the circuit performance to variations in the line lengths and device placement. Flip-chip mounting has been primarily used at lower frequencies, at which slight variations in the device mounting location is not important to the circuit performance. In the microwave/millimeter wave range, however, the accuracy of device placement and attachment is highly important to the achievement of low cost circuits and systems. Sakai et al. achieved a chip alignment accuracy of 5.5 micrometers for transmission line widths of only 16 micrometers; such a low placement accuracy with respect to the line width is believed to be unacceptable at millimeter wave frequencies for achieving reproducible circuit performance.
In contrast to the relatively high loss and expensive silicon substrate employed by Sakai et al., Duroid substrates or other similar plastic type substrates have been developed which have both a lower cost and a lower loss level. Duroid is a trademark of Rogers Corporation for a doped Teflon.RTM. composition (the chemical formula for Teflon is PTFE). Low-loss plastic type substrates are available inexpensively with metalization on both sides, and do not require the deposition of SiO.sub.2 and the two metal layers of Sakai et al. to fabricate low-loss transmission lines. However, Duroid substrates are relatively soft and it is therefore difficult to wire bond discrete devices onto them. At lower frequencies these substrates are used for microwave circuits that employ previously packaged components, such as discrete transistors that are wire bonded inside a package which in turn is mounted on the Duroid substrate. However, the package parasitics of the components are too high for this approach to work at millimeter wave frequencies. Also, up to a thickness of about 600 micrometers Duroid substrates are too flexible for reliable automated flip-chip mounting; MMICs are therefore conventionally flip-chip mounted on very hard substrates, such as alumina. Thus, despite their desirable low cost, Duroid substrates have not been suitable for flip-chip mounting of high frequency MMICs.